Embodiments of the present disclosure relate to an ink-jet printing method and a manufacturing method of an OLED display device.
In the fields of Organic Light Emitting Diode (OLED) and Liquid Crystal Display (LCD), methods such as ink-jet printing method are usually used to form a thin film made of an organic functional layer or a color film structure, wherein the ink-jet printing method is one of wet film forming processes. Such a wet film forming process generally comprises an indispensable step of removing unwanted solvent by subsequent process, thereby drying and forming the desired thin film. This drying process for removing the solvent determines the topography of the formed thin film. For a photoelectric display device, whether the topography and uniformity of the dried thin film is good or bad, has a great influence on the lifetime and display effect of the device.
FIG. 1 is a schematic principle diagram for explaining the process in which an ink droplet dries on a flat surface to result in coffee ring phenomenon. After an ink droplet drops into a substrate or a pixel, due to the shape of the ink droplet, i.e., due to the formation of the surface tension change curve A, the central portion of the ink droplet with smaller volume and unit areas has a different evaporation rate than the edge portion with larger volume and unit areas. Thus, the drying rate of the ink in the edge portion is faster. As the solvent in the edge portion continuously evaporates, the solid content distribution in the central portion becomes different from that in the edge portion, such that there exists a concentration gradient (difference) between the central and edge portions, thereby causing the capillary compensation phenomenon that the solvent flows from the central portion to the edge portion. The flow direction C of the capillary compensation is shown in the figure. Such flow of the solvent certainly will drive part of the solute to migrate to the edge portion, such that the amount of the solute in the edge portion continuously increases, finally resulting in the formation of the coffee ring effect. In addition to the above-mentioned capillary compensation flow from the center to the edge of the ink droplet, there exists inside the ink droplet a Marangoni convection phenomenon in the inverse direction. The direction B of the Marangoni convection is shown in the figure. The dried topography of the final thin film is the result decided by both these micro-flow effects. The study found that the root cause of the Marangoni convection phenomenon is the surface tension gradient (difference) between different parts inside an ink droplet. Many factors such as temperature gradient, concentration gradient, change of solvent composition, and so on, may cause the generation of this surface tension gradient (difference).
In addition, the formation of the coffee ring effect requires a condition, i.e., the pinning effect of the contact line between the ink droplet and the interface. The moving and pinning of the contact line is a complex process influenced by ink characteristics (such as boiling point, viscosity, surface tension and the like), lyophilic and lyophobic properties of the interface material, the magnitude of the interfacial tension between liquid and solid, and the like. FIG. 1(b) shows a cross-sectional view of a thin film formed after the droplet having the coffee ring effect dries on the flat surface. It can be seen that after the droplet is dried, the thickness of the edge portion is significantly greater than that of the center. This results from the combined action of the interfacial pinning effect and the solute migration due to the Marangoni convection phenomenon inside the droplet during the drying process.
In the actual preparation of a display device, the shape of a pixel is the shape of its boundary defined by a layer of so-called pixel defining layer material. With respect to the solution preparation process such as ink-jet printing process, there are some additional requirements for the pixel defining layer. One requirement is that the thickness may be great (typically 1-2 μm) for storing the printed ink during the preparation process. Another and more important requirement is that the surface of its material must have strong lyophobic property, to avoid the mixing of ink between adjacent pixels, and at the same time to make the falling ink droplets with slight offsets (i.e., the falling ink droplets dropped on the inner edge of the pixel) roll into the pixel pit.
The usage of the pixel defining layer can increase the pinning effect of the contact line, such that in the entire drying process, the contact line is always fixed at the edge of the pixel defining layer, thereby increasing the coffee ring effect. In particular, only a very thin layer at the surface of the currently used pixel defining layer material has lyophobic property, while the lower layer occupying its majority is lyophilic. FIG. 2 is a schematic cross-sectional view showing the topography where the ink droplet drops into a groove defined by the defining pixel layer. FIG. 3 is a schematic cross-sectional view showing the contour of the thin film actually measured after drying. It can be seen from FIG. 3 that most of the organic functional material 3 (such as hole injection layer material) is located at the portion within the pixel close to the edge 4 of the pixel defining layer, such that it is difficult to guarantee the flatness of the thin film, resulting in uneven luminance of the device as well as great decrease in utilization efficiency of the material.
It can be known from the above introduction that several conditions should be met to form the coffee ring effect. One condition is the Marangoni convection phenomenon during the evaporation process of the solvent. Another condition is the pinning effect of the solute during the drying process. To reduce the coffee ring effect, the existing technical solutions mainly aim at changing the Marangoni convection phenomenon, such as by adding ellipsoidal particles into the ink, by adjusting the pH value of liquid, by adding a second component solvent or additive, and by electrowetting method and the like. For example, Kajiya Tadashi et al. (Controlling the Drying and Film Formation Processes of Polymer Solution Droplets with Addition of Small Amount of Surfactants) improved the Marangoni convection phenomenon by adding a small amount of surfactants into the ink to change the surface tension difference between different parts of the ink droplet during the drying process, thereby achieving the effect of improving the flatness of the dried thin film. In general, the above methods can play a certain role of reducing the coffee ring effect. However, for an OLED, the additives such as surfactants often result in the decrease in device performance, and thus are difficult to use in the OLED structure.
Furthermore, all of the above methods are solutions targeted for droplet spreading and drying processes on a flat surface. There are rarely methods suitable for pixelated drying process. That is, the influence of properties of the pixel defining layer on the topography of the dried thin film has not been considered. As shown in FIG. 2, the pixel defining layers used by existing ink-jet printing processes mostly have a two-layer structure. The first defining layer 1 is lyophilic, and the second defining layer 2 is an organic resin structure. Only a thin layer 202 at the surface of the second defining layer 2 has lyophobic property, while the lower part 201 occupying most of the thickness still has lyophilic property. This results in that when the printed organic functional ink is drying, the contact line is fixed at the lyophilic-lyophobic interface of the second defining layer 2. That is, the pinning effect occurs. Then, due to the coffee ring effect, the serious edge climbing phenomenon occurs that the thin film exhibits a thin center and a too thick edge. FIG. 3 shows the topography of the thin film formed after drying. This is a specific exhibition of the coffee ring effect inside a pixel having a pixel defining layer.
Therefore, based on the material and structural properties of existing pixel defining layers, after OLED functional material (such as hole injection layer material) is ink-jet printed into a pixel, the serious climbing phenomenon will occur that the formed thin film exhibits a thin center and a too thick edge due to the coffee ring effect during the drying process.